Induction motors consist generally of a stator and a rotor, both made of ferromagnetic material of high permeability. A set of coils embedded in the stator is fed by multi-phase currents to produce a rotating magnetic field. Depending on the geometric layout of the coils and on the current in them, different configurations of the magnetic field in the motor may be produced. Because of Faraday's law, the stator magnetic field causes an EMF to be induced on the rotor which generates current in the rotor. This current interacts with the magnetic field and produces a torque that rotates the rotor.
As long as the rotor is rotating at a speed lower than the rotation speed of the magnetic field, the induced EMF is always there to produce current and torque. However, if the rotor catches up with the rotating magnetic field, the relative motion between the field and rotor and the induced EMF disappear and no force is produced. Consequently, without the applied force, the rotor slows down.
In order to prevent situations where the rotor catches up to the rotating magnetic field, the rotor is typically configured to rotate at a speed slightly less than the speed of the rotating magnetic field. The difference between the magnetic field speed and rotor speed is typically called slip. For example, an AC induction motor having a standard 120 v, 60 Hz, 3-phase constant AC input generally produces a magnetic field rotating at 1800 RPM. In this situation, the rotor is typically configured to run at about 1725 RPM when no load is applied to the rotor. Therefore, the no-load slip is 1800 RPM-1725 RPM or 75 RPM. However, if a load is applied to the motor rotor, the rotational speed of the rotor will slow down thus increasing the slip.
A motor is running most efficiently when the slip is relatively small. Thus, it is desirable to provide a slip control means for controlling the slip and maximizing the motor's efficiency. One way to provide slip control is to control the magnetic field rotational speed by providing a variable frequency input to the motor stator. In this situation, the frequency of the variable frequency input is typically called the slip control frequency.
Prior to the development of the present invention, variable frequency control was typically accomplished by using transducers to relate the rotor position to an electronic controller which in turn calculates the specific vector functions and supplies variable frequency motor currents based on each specific motor design. The electronic controllers of the prior art are complex and can become unreliable due to their complexity. Furthermore, existing electronic controllers must be modified for each motor to accommodate different motor performance characteristics.
U.S. Pat. No. 3,614,577 issued to Honeywell et al. discloses a synchro-servo system in which motor torque is transmitted directly from the motor to the synchro rotor without interposed gearing. A synchro and a servomotor in a common housing share a common shaft. When an input from a synchro transmitter causes a magnetic field to be produced by a Y-connected synchro input winding, a current is generated in the synchro secondary winding. This current is amplified by an amplifier and fed to a control phase winding of the servomotor. The resultant torque produced by the servomotor rotates the rotor shaft to a new null position.
U.S. Pat. No. 3,569,782 issued to Shalihi et al. discloses a system having a disk of nonmagnetic material that includes a number of ferrite bars radially aligned in pairs. The disk is coupled to the rotor shaft of an AC induction motor to be controlled. A second disk is mounted in proximity to the first disk and is provided with multiple sets of imbedded C-shaped ferromagnetic elements. A step-transformer is mounted on an insulative bobbin by an auxiliary speed-controlled motor.
As the ferrite elements periodically align with one another, as a result of relative disk rotation, the reluctance of the magnetic field of an interposed coil changes periodically and results in amplitude modulation of a carrier signal applied to the coil primary by an oscillator. The frequency of this modulation is proportional to the sum (or difference) of the speeds of the two disks.
In effect, the auxiliary motor is a slip-frequency determining motor that controls the slip frequency of an induction motor. The rotor speed derived from the first disk is converted to a rotor frequency, the slip frequency is combined to provide the synchronous frequency, and this frequency is multiplied to provide the frequency of pulses required by the inverter.
U.S. Pat. No. 3,600,692 issued to McGee discloses a system designed to control motion of machine tool elements in numerical control systems. In the preferred embodiment, a sine/cosine resolver is used to provide instantaneous position information in the form of a single sinusoidal reference wave. A phase discriminator derives a final DC signal for driving a servoamplifier and servomotor that properly positions the machine tool element.
U.S. Pat. No. 4,330,741 issued to Nagase et al. discloses a field oriented control apparatus for an induction motor in which the exciting current and the secondary current are independently controlled to control the amplitude, frequency, and phase angle of the primary current in order to adjust motor torque. In the control apparatus, changes in secondary resistance are detected based upon deviation in the output of a primary voltage setter and a primary voltage detector, and the slip frequency is corrected based upon these deviation values. Thus, primary voltage and torque of the induction motor can be controlled without being unduly influenced by changes in secondary resistance.
U.S. Pat. No. 4,357,569 issued to Iwakane et al. discloses a vector controller for an AC induction motor designed to provide a torque equivalent to that of a DC motor by controlling the instantaneous values of stator current in the AC motor. Signals from a brushless position detector designed for high temperature operation (such as a resolver or a pulse generator coupled to the AC motor) are used to generate instruction signals for the stator current, and are also used to derive rotational position and speed feedback signals for precise servo control.
In one embodiment, the patent describes the derivation of a current instruction signal and a position signal by subjecting the resolver output signal to frequency division, and the derivation of a speed feedback signal through frequency-to-voltage conversion of the resolver signal, such that AC frequency and current supplied to the synchronous motor are controlled by these three signals.
U.S. Pat. No. 5,066,899 issued to Nashiki discloses a vector controller for an induction motor in which the vector controller comprises a secondary current detection unit designed to detect secondary current signals from the induction motor and a slip compensation unit that determines a slip frequency compensation value based upon the secondary current detection unit. A slip frequency generating unit determines slip frequency from the secondary current command. The field speed command for the induction motor is then based upon the slip frequency compensation value, the slip speed, and the motor velocity.
U.S. Pat. No. 5,293,445 issued to Carbolante discloses a phase-locked loop (PLL) motor control system having a variable gain such that a relatively low gain is applied when the phase error between the reference frequency and the variable frequency (the motor speed) is relatively large, such as during convergence. As the phase error decreases, such as when lock is imminent, the gain value is increased.
While the above-mentioned devices may work for their intended purposes, none of these devices accomplish slip control through use of a variable frequency feedback transducer wholly within the motor without the need for additional electronic control circuitry used to calculate field vectors and compute frequencies for slip control. It is thus apparent that there is a need for an improved method and apparatus for modulating the slip control frequency.
An object of the present invention is to provide a variable frequency feedback transducer which can be contained within the motor to automatically maintain a constant frequency difference between the rotor frequency and the magnetic field frequency.